1. Field of the Invention
The present invention relates to a print position adjusting method of adjusting a drive timing for a print head, and more specifically, to a print position adjusting method of adjusting print positions for forward and backward scans as well as an ink jet printing apparatus and an ink jet printing system both using this method.
2. Description of the Related Art
Relatively inexpensive OA equipment such as personal computers and word processors has been popularized in recent years. Various output apparatuses such as printing apparatuses have been provided which output information created by such equipment. In particular, printing apparatuses are very popular, and methods of increasing the printing velocity of these apparatuses and techniques of improving image quality have been developed rapidly.
Further, among these printing apparatuses, especially, serial printers using an ink jet printing method receive much attention because they achieve high velocity printing or can print high quality images without requiring high costs.
For example, a bidirectional printing method is a technique of allowing a serial printer to achieve high velocity printing. For example, a multipass printing method is a technique of printing high quality images.
To increase printing velocity, it is contemplated that a printing operation may be performed using a print head having an increased number of print elements. However, this method results in an increase in the size of a print head. The bidirectional printing, in which a print head carries out printing during both forward and backward scans, is an effective method for increasing the printing velocity without increasing the size of the print head.
Although a simple proportional relationship is not established because printing apparatuses normally require time for sheet feeding and discharging and the like, the bidirectional printing substantially doubles the printing velocity compared to unidirectional printing that carries out printing only during a forward scan.
For example, it is assumed that a print head is used which has a printing density of 360 dpi and which has 64 ejection openings arranged in a direction different from a main scanning direction (for example, a sub-scanning direction, in which print media are fed) and that A4-sized print media are fed in their longitudinal direction for printing. In this case, the print head must execute about 60 printing scans in order to print images all over the print medium. In the unidirectional printing, all the printing scans are carried out during movement in only one direction from a predetermined scan start position. This printing method also involves non-printing scans in the opposite direction for returning from a scan end position to the scan start position. Consequently, about 60 reciprocatory scans are required in order to print images all over the print medium under the previously described conditions. On the other hand, in the bidirectional printing, a printing operation is performed during both forward and backward scans. Consequently, the entire image can be completed by executing about half or 30 reciprocatory scans. Thus, the bidirectional printing sharply reduces the time required for printing. This enables the printing velocity to be improved.
Now, description will be given of the multipass printing method as an example of a technique of improving image quality. If a printing operation is performed using a print head having a plurality of print elements, the grade of printed images depends markedly on the performance of the sole print head. For example, with an ink jet print head, the amount of ink ejected from ejection openings or the direction of the ejection may be affected by a small manufacturing error that may occur during a print head manufacturing process, such as differences among manufactured elements used to generate energy utilized to eject ink, such as electrothermal converters, i.e. ejection heaters. Consequently, the resulting image may have a nonuniform density and thus a reduced grade.
A specific example will be described below with reference to FIGS. 10A to 10C and FIGS. 11A to 11C. In FIG. 10A, reference numeral 901 denotes a print head that is assumed to be composed of eight nozzles 902 for simplification (in the specification, the term “nozzle” generally refers to an ejection opening, a liquid passage in communication with the ejection opening, and an element that generates energy utilized to eject ink, unless otherwise specified). Reference numeral 903 denotes ink ejected from the nozzle 902 as, for example, a droplet. Ideally, an almost equal amount of ink is ejected from each ejection opening in the same direction as shown in FIG. 10A. If such ejection is successfully achieved, ink dots of an equal size impact a print medium as shown in FIG. 10B to form a uniform image with a generally uniform density as shown in FIG. 10C.
However, in actuality, the individual nozzles in the print head 901 are different as described previously. Thus, if the print head 901 is used for printing as it is, the size or direction of an ejected ink droplet varies among the nozzles as shown in FIG. 11A. As a result, ink droplets impact a print medium as shown in FIG. 10B. This figure shows that in the head main scanning direction, blank portions having an area factor of less than 100% may appear periodically or conversely dots may overlap one another more than required or a blank line may appear as seen in the center of the figure. If dots impact the print medium in this condition, they have the density distribution shown in FIG. 11C in the direction in which the nozzles are arranged. As a result, human eyes perceive these phenomena as the nonuniformity of the density.
Thus, the multipass printing method has been devised in order to avoid the nonuniformity of the density. This method will be described below with reference to FIGS. 12 and 13.
The print head 901 is scanned three times as shown in FIG. 12A in order to print completely an area similar to that shown in FIGS. 10A to 10C and 11A to 11C. Two passes are used to complete an area composed of four pixels, the half of eight pixels, arranged in the vertical direction of the figure. In this case, the eight nozzles in the print head are grouped into two sets each including upper or lower four nozzles. Dots formed by one nozzle during one scan are determined by decimating image data to about half in accordance with a predetermined image data sequence. Then, during the second scan, dots are filled into the remaining half of the image data to complete the area composed of four pixels.
The multipass printing method reduces the adverse effect of differences among manufactured nozzles on printed images even with the print head 901, shown in FIG. 11A. A printed image is as shown in FIG. 12B, and such overlapping or blank lines as shown in FIG. 11B are not very marked. Consequently, as shown in FIG. 12C, the nonuniformity of the density is substantially suppressed compared to FIG. 11C.
When such multipass printing is carried out, image data is divided into two pieces for the first and second scans in accordance with a predetermined arrangement, i.e., a mask so that these pieces are complementary to each other. In the most common case, in this image data arrangement, i.e., a decimation pattern (thinning out pattern), each pixel for the first scan alternates with the corresponding pixel for the second scan in both vertical and horizontal directions. In a unit print area (in this case, composed of four pixels), the entire image is printed by the first scan for forming every other dot and the second scan for using a pattern opposite to that for the first scan to form dots. Further, the distance a print medium moves during each scan, i.e., the amount of sub-scan, is set at a specified value. In FIGS. 12 and 13, the print medium is moved a distance equal to four nozzles during each scan.
The multipass printing method is particularly effective in printing an image with a relatively high print duty such as a solid image in which the nonuniformity of the density or a blank line, if any, is visually perceived easily. However, in texts or ruled lines, which have a relatively low print duty, it is difficult to perceive the nonuniformity of the density or a stripe, if any. Accordingly, the multipass printing cannot be advantageously executed on these images. Thus, it is contemplated that in printing texts or ruled lines, the multipass printing is not carried out, while priority is given to a high printing velocity.
A registration technology of adjusting the impact positions of dots is another example of a technique of improving image quality in a dot matrix print method. The registration is a method of adjusting a position on a print medium at which a dot is formed, by for example, changing a drive timing for a print head.
Ink droplets ejected from the nozzles may impact at positions different from target ones owing not only to the varying ejection characteristics of the individual nozzles but also to the factor of the average head ejection characteristics or the mechanical factor of the main body. For example, the distance between each head nozzle and a print medium (paper distance) varies slightly among individual printing apparatuses because of manufacturing errors. A variation in paper distance results in a variation in the time required by ink droplets ejected from the nozzles to impact the print medium. This may vary the impact position during bidirectional printing. The same phenomenon may result from a variation in ejection velocity caused by differences among manufactured heads.
FIGS. 14A to 14E show an example of a variation in the impact position.
As shown in FIG. 14A, it is ideal for an ink droplet to impact a print medium at the same position during both forward and backward scans. However, if there is a large distance between each nozzle and the print medium, the impact position varies between a forward scan and a backward scan because the print medium is located below the intersection between the track of an ink droplet during the forward scan and the track of an ink droplet during the backward scan as shown in FIG. 14B. In contrast, if there is only a small distance between each nozzle and the print medium, the impact position varies between the forward scan and the backward scan because the print medium is located above the intersection between the track of the ink droplet during the forward scan and the track of the ink droplet during the backward scan as shown in FIG. 14C.
Further, if the ejection velocity is high, the ink droplets impact the print medium before their tracks meet as shown in FIG. 14D. On the other hand, if the ejection velocity is low, the ink droplets impact the print medium after their tracks have met as shown in FIG. 14E. In this manner, there are various factors relating to a variation in the impact position between the forward scan and the backward scan.
Further, if an image is formed using plural rows of nozzles, the impact position may vary owing to differences in average ejection characteristics (ejection direction and velocity) among the nozzle rows. Such a variation in the impact position may degrade images. Therefore, the registration is an essential technique for improving the image quality.
The registration is generally carried out as follows:
For example, in reciprocatory printing, to align the impact position during the forward scan with the impact position during the backward scan, ruled lines or the like are printed on a print medium while varying a relative print position condition between the forward scan and the backward scan, in order to adjust print timings for the forward and backward scans, respectively. An inspector visually checks the printed ruled lines to select conditions under which the impact position during the forward scan is aligned with the impact position during the backward scan, i.e. conditions under which ruled lines or the like are printed without being misaligned. The inspector then sets the impact position conditions in the printing apparatus by inputting them directly to the printing apparatus by key operations or the like or by operating a host computer using an application.
Further, if a printing operation is performed using a print head having plural rows of nozzles, the individual nozzle rows are used to print the respective ruled lines or the like on a print medium while varying relative print position conditions among the plurality of nozzles. Then, as described previously, the user selects optimum conditions under which the print position does not vary. Then, the inspector uses means similar to that described previously to set print position conditions in the printing apparatus so that different relative print position conditions are set for the respective nozzle rows.
In recent years, efforts have been made to increase the definition of ink dots, i.e. reduce the size of ink droplets in order to improve the image quality achieved by the ink jet printing apparatus. Accordingly, a small variation in the impact position or the like, which is unnoticeable in the case of conventional large dots, is now noticeable owing to the reduced size of dots. Therefore, in connection with an ink ejecting operation performed by the print head, not only the conventional registration but also the phenomena described below must be taken into account.
As a first phenomenon, main-droplet and satellite impact positions vary between the forward scan and the backward scan.
FIG. 15 is a schematic view showing the structure of a print head and ejected ink droplets.
For example, if the print head is adapted to eject ink on the basis of a BUBBLE JET ® method, thermal energy from a heater 1401 is used to generate bubbles in ink so that pressure generated by the bubbles causes the ejection of a predetermined amount of ink droplet present close to an ejection opening 1402. However, liquid-liquid separation, i.e., the separation of the ink droplet from the nozzle, is unstable. Accordingly, after a main droplet 1403, an ink droplet called a “satellite” 1404 is ejected. The satellite 1404 is formed by separating the trailing end of the ejected droplet from its remaining part. The satellite 1404 has a smaller volume and a lower ejection velocity than the main droplet 1403. Further, the satellite 1404 is generated whether the BUBBLE JET ® method or a piezoelectric method or the like is used as an ink ejecting method.
As shown in FIG. 16, the main droplet and the satellite fly in the same direction. However, since the print head carries out printing while moving in the main scanning direction, the main droplet and the satellite impact at different positions owing to a difference in ejection velocity between them. Using a main-droplet ejection velocity V, a satellite ejection velocity v, paper distance D, and a print head scanning velocity Vp, the distance L between the impact positions of the main droplet and satellite can be expressed as follows:L=Vp×(D/v)−Vp×(D/V)
In this manner, dots 1501 and 1502 are formed on the print medium by the main droplet and the satellite, respectively. However, if the main droplet dot and the satellite dot are sufficiently small, it is possible to consider only the main droplet to contribute to printing while neglecting the adverse effect of the satellite.
However, as described above, as the size of ink droplets and thus the size of the main droplet decrease, it becomes impossible to neglect the adverse effect of the satellite. That is, the volume of the satellite relates closely to ejection characteristics determined by the shape of the nozzles or the like. Thus, it does not decrease consistently with the size of the main droplet. Accordingly, as the size of the main droplet dot decreases, the difference in size between the satellite dot and the main droplet dot tends to decrease. Specifically, the leading end of the ejected droplet becomes the main droplet, whereas the separated trailing end becomes the satellite dot. Thus, the characteristics of the ejection port or ink, specifically viscosity and surface tension, affect the size of the satellite dot. Accordingly, even if the size of the main dot is reduced, the size of the satellite dot does not decrease in proportion to the reduction in the size of the main droplet. As a result, a decrease in the size of droplets relatively enhances the adverse effect of the satellite dot. Therefore, it is important that an image forming technique takes even the satellite into consideration.
An example is given in which ruled lines are printed in the vertical direction (sub-scanning direction). Description will be given using a head having 304 nozzles arranged at a pitch of 600 dpi.
If bidirectional printing is carried out, the positional relationship between main droplet dots and satellite dots is reversed between a forward scan and a backward scan.
FIG. 17A is a schematic view showing the positions of main droplet dots and satellite dots observed if bidirectional printing is carried out in non-multipass printing. FIG. 17B is an enlarged schematic view of a part of the main droplet and satellite dots corresponding to one scan.
If one-pass printing, i.e., non-multipass printing, is carried out, a forward scan and a backward scan are switched at intervals of 304-nozzle widths (about 13 mm). Accordingly, the results of printing are such that the positions of the satellite dots are reversed at intervals of about a 13-mm width.
FIG. 17C shows the line density of printed ruled lines. For example, when the main droplet ejection velocity V=15 m/s, the satellite ejection speed=10 m/s, the paper distance D=1.6 mm, and the scanning velocity Vp=25 inch/s, the length of misalignment L is 0.03 mm. Since human sense of sight is characterized by having a low resolution, the ruled lines are substantially perceived as the line density schematically represented in FIG. 17D. Between a forward scan and a backward scan, the line density is reversed as shown in FIG. 17E. The line density during the forward scan does not substantially overlap the line density during the backward scan. Accordingly, the results of printing are such that parts of a ruled line each corresponding to the nozzle width are connected together irregularly. To join together smoothly parts of the ruled line printed during forward and backward scans, respectively, the print head must be registered as shown in FIG. 18A to maximize the overlapping of the line densities of forward and backward scan dots.
On the other hand, if multipass printing is carried out, forward printing and backward printing are equally executed around pixels. Consequently, satellite dots are almost equally formed at the right and left sides of main droplet dots (see FIG. 19A). Then, since human sense of sight is characterized by having a low resolution, the line density shown in FIG. 19B is substantially perceived. Thus, to print ruled lines smoothly, the print head must be registered so that the main droplet dots constitute the same column.
As described above, if the adverse effect of satellite dots is not negligible, the optimum registration value varies between multipass printing and non-multipass printing. Furthermore, the length of misalignment L increases in proportion to the moving velocity of the print head. Consequently, if the print head is moved fast in order to increase the velocity of the printing apparatus, the distance L between a main droplet dot and a corresponding satellite dot increases. This makes the satellite dot noticeable, and the problem becomes more serious.
Next, a second phenomenon will be described. It is assumed that a plurality of driving motors are used which use different time intervals at which ink is ejected (for example, a 1,200-dpi mode and a 600-dpi mode) and that registration is carried out on the basis of an ink ejection timing in one of the driving modes. Then, if a printing operation is performed in the other driving mode, the impact positions of dots during a forward scan may be slightly misaligned with respect to the impact positions of dots during a backward scan. This misalignment is noticeable owing to the reduced diameter of the dots.
A block division driving method has hitherto been known which is used in driving a print head with a plurality of nozzles to eject ink, in order to reduce the power supply capacity required for driving: the method comprises dividing a group of nozzles into a plurality of blocks and driving these blocks simultaneously so as to eject ink.
FIGS. 20 to 22 show ejection timings for the respective nozzles used if the block division driving method is used to eject ink from the nozzles in accordance with print data. As shown in FIG. 20, for example, 304 nozzles in a head are divided into a plurality of blocks (in this case, 19 blocks). Then, the ejection order of the nozzles in each block is specified as shown in FIG. 21. Then, ejection is carried out in accordance with the pulse timings shown in FIG. 22. That is, at one point in time, ink is ejected from the nozzle corresponding to the ejection order 1 in all the blocks. Then, a time d later, ink is ejected from the nozzle corresponding to the ejection order 2 in all the blocks. Similarly, ejection is sequentially executed on the nozzles corresponding to the ejection orders 3 to 16 using sequentially delayed timings.
The control based on the block division driving enables the number of simultaneous ejections to be reduced. This makes it possible to prevent an excessive current from being instantaneously generated compared to the simultaneous driving of all the nozzles.
However, with the above method, the respective nozzles within each block use different ejection timings. Accordingly, the impact position varies slightly depending on the nozzle. Specifically, if a CR velocity is 151 inch/sec and the delay time d is 3.5 μsec and if an attempt is made to print a ruled line parallel with the nozzle rows, then a ruled line actually obtained is shifted from the parallel position by 1/1,200 inch (about 21 μm) as shown in FIG. 23. This phenomenon may degrade images. Thus, in order to reduce the shifting width w shown in FIG. 23, it is desirable to minimize the delay time d for the drive timing.
Ink jet printers normally employ a method of ejecting ink from the nozzles by exerting pressure on the ink on the basis of bubbling caused by film boiling on heaters or the vibration of piezoelectric elements. The pressure propagates not only to the front of each nozzle (in ejecting direction) but also to its rear, i.e. to the inside of a liquid chamber. The pressure propagated to the liquid chamber further propagates to surrounding nozzles. As a result, the ink in nozzles present close to the nozzle from which ink has been ejected is vibrated. When pressure is exerted while the ink is being vibrated, ink may not be correctly ejected owing to the unstable state in the nozzles. Thus, after ejection, the next ejection must be started after a pause corresponding to the time required to stop the vibration. With a small number of simultaneous ejections, only a low pressure propagates to surrounding nozzles. Accordingly, the vibration of the ink in a nozzle is stopped in a relatively short time.
In multipass printing, the number of ejections per scan normally decreases with an increase in the number of passes (the number of scans required to complete an image occupying a predetermined area). Specifically, in printing with a large number of passes, the number of simultaneous ejections is relatively small. Consequently, the adverse effect of the pressure propagation is not substantially produced, thus allowing the delay time d for the drive timing to be reduced. In contrast, in printing with a small number of passes, the number of ejections is relatively large. Consequently, the above adverse effect is produced, thus requiring the delay time d for the drive timing to be extended. Thus, some printers having a plurality of print modes with different number of passes carry out printing using a plurality of drive modes with different delay times d for the drive timing.
However, the dot shifting width w varies depending on the drive mode. Thus, if reciprocatory printing is carried out using the same reciprocatory registration value in spite of different drive modes, the impact position may vary between a forward scan and a backward scan. This will be described below with reference to the drawings.
FIGS. 24A and 24B are schematic views showing an arrangement of dots on a sheet in order to describe a phenomenon in which when a checker-pattern-like mask is used for two-pass printing, the impact position varies during bidirectional printing because of different drive modes. FIG. 24A shows a drive mode in which the delay time d for the drive timing is set at 3.5 μsec in order to reduce the dot shifting width w to 1,200 dpi ( 1/1,200 inch) (this mode is called a “1,200-dpi drive mode”). This figure shows, in its left, the positions of dots obtained during the first and second scan ejections, and in its right, the arrangement of the dots on a sheet after printing. The scanning direction is reversed between the first scan and the second scan. Accordingly, before the second scan, i.e., before backward printing, the ejection order within each block is reversed.
FIG. 24B shows a drive mode in which the delay time d for the drive timing is set at 7.0 μsec in order to reduce the dot shifting width w to 600 dpi ( 1/600 inch) (this mode is called a “600-dpi drive mode”) This figure shows, in its left, the positions of dots obtained during the first and second scan ejections, and in its right, the arrangement of the dots on a sheet after printing.
For both printing operations, the reciprocatory registration value is adjusted so that the optimum impact position is obtained in the 1,200-dpi drive mode.
In each drive mode, the ejection order within each block is reversed between a forward print scan and a backward print scan in order to deal with reciprocatory printing.
As seen in FIG. 24B, when 600-dpi driving printing is executed with the reciprocatory registration value set so as to obtain the optimum impact during 1,200-dpi driving, the impact position is misaligned with respect to the optimum one because the dot shifting width in this drive mode is different from that in the 1,200-dpi drive mode.
If dots of a large diameter are formed on a medium when ink impacts it, the adverse effect of the impact misalignment is relatively insignificant. Accordingly, the degradation of images is of the level at which it is not perceived. However, as the size of ink droplets decreases to reduce the dot diameter the adverse effect of the impact misalignment becomes so significant as not to be negligible.
As described above, as the size of ejected ink droplets decreases to reduce the diameter of printed dots, the adverse effect of a variation in the impact position between a forward scan and a backward scan becomes significant depending on the drive mode of the block division driving. Thus, disadvantageously, the degradation of images is noticeable.